The present invention relates to a speech scrambler, and more particularly to a speech scrambler which employs are orthogonal transformation technique, such as fast Fourier transform (FFT) or fast Hadamard transform.
Widely known speech scramblers are roughly divided into those which involve signal processing in a frequency domain, such as a frequency inversion method and a scrambler which divides a voice signal into a plurality of frequency slots and then rearranges the slots with or without frequency inversion in each slot, and those which involve signal processing in the time domain, such as a system which sections a voice signal into blocks in terms of time and changes the order of sample values in each block or inverts the sign of each sample value.
In recent years there have also been proposed a scrambler which combines signal processing in frequency domain and time domain, and a scrambler which utilizes an orthogonal transform.
A prior art example of this kind is disclosed in Japanese Pat. Disc. Gazette No. 153862/81. This is a scrambler which rearranges frequency spectrums obtained by a fast Fourier transform or a fast Hadamard transform of a voice signal in accordance with a predetermined rule, and transmits the time domain signal after the inverse transform.
With the prior art system, the rule for rearranging the spectrum is predetermined by the scrambling key and the number of keys available is so large that even if a scrambled telephone signal is wiretapped, it would be difficult to detect the spectrum rearrangement rule employed and descramble as the original voice signal; hence this system seems to ensure the security of communication. However, since the security function of this system depends on how to rearrange the spectrum of the original voice signal, the total amount of energy remains unchanged before and after the operation. Consequently, the scrambled signal produced by the operation still retains the intonation of the original speech. Thus, the intensity of the original speech and unvoiced silent period therein can be readily detected. For instance, even if such a scrambler is employed in a communication circuit for stereotyped conversations, their contents might be understood to some extent by experience. Even if the contents of communication cannot be directly understood from the scrambled voice, voiced sections to be deciphered can easily be located. Therefore, the conventional speech scrambler possesses such a serious drawback that its security is not necessarily satisfactory.
In addition, the prior art merely rearranges the spectrum of a voice signal, and hence does not effectively serve the purpose for voices of little energy, for example, at the beginning and the end of a speech and for a voice of little energy and flat spectrum, such as a fricative sound.